Answer: Some darkening of the seed heads is normal however it should not look like a fungus on the whole plant. I would trim them back very short and clean my clippers off between plants by dipping them in a bleach and water solution. Then I would carefully remove all the trimmings and either put them in the trash or burn them -- do not compost them. To be honest I doubt the birds would eat seed if it has gone bad in some way anyway. In the spring, keep an eye out for signs of the same thing happening. Next spring, you might also consider dividing them and spreading them a bit further apart for better air circulation if they are either several years old or crowded thickly together. This should help reduce the chance of infection next year.
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